Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session M14: Free Surface Flows: General |
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Chair: Alfredo Soldati, University of Udine Room: C125-126 |
Tuesday, November 22, 2016 8:00AM - 8:13AM |
M14.00001: Experimental studies of the streaming flow due to the adsorption of particles at a liquid surface Pushpendra Singh, Naga Musunuri, Ian Fischer The particle image velocimetry (PIV) technique is used to study the streaming flow that is induced when particles are adsorbed at a liquid surface. The flow develops within a fraction of second after the adsorption of the particle and persists for several seconds. The fluid directly below the particle rises upward, and near the surface, it moves away from the particle. The flow causes powders sprinkled on a liquid surface to disperse on the surface. The flow strength, and the volume over which it extends, decreases with decreasing particle size. The streaming flow induced by the adsorption of two or more particles is a combination of the flows which they induce individually. [Preview Abstract] |
Tuesday, November 22, 2016 8:13AM - 8:26AM |
M14.00002: Steady streaming in standing waves Jean Rajchenbach, Enrica Saggese, Didier Clamond We report the existence of recirculating eddies existing in the bulk of a liquid under the action of standing surface waves. This phenomenon results from the combined action of the nonlinearity and viscosity. The period of these secondary flows can be, say, one hundred times that of the wave, depending on the amplitude. Our experimental results reveal strong disagrements with theoretical predictions devised hitherto. In order to account for our data, we propose a new mechanism playing a major role in the formation of these rolls. [Preview Abstract] |
Tuesday, November 22, 2016 8:26AM - 8:39AM |
M14.00003: Experimental investigation of the interaction between turbulent boundary layers and near-surface wave-induced forcing. Owen Williams Free-surface waves can have a significant impact on sub-surface turbulent boundary layers that are present on undersea vehicles or on the bottom of flowing bodies of water such as estuaries. This problem has a wide parameter space and resultant changes to boundary layer structure due to wave forcing still require investigation. Here, preliminary experimental measurements within the newly commissioned wave channel at the University of Washington are detailed. Particle image velocimetry (PIV) is used to examine velocity statistics across the water column. In an effort to more readily identify changes in underlying boundary layer structure, a range of flow decompositions, such as snapshot partial orthogonal decomposition (POD) are evaluated in an effort to separate turbulent motions from the forcing, which to first order is a traveling wave. The effect of the relative difference between water depth and boundary layer thickness will be examined, as well as the Froude number of the surface waves. Ongoing efforts to examine the full parameter space will be discussed, as dimensional analysis and linear wave theory suggest there are up to seven parameters relevant to either inner or outer layers. [Preview Abstract] |
Tuesday, November 22, 2016 8:39AM - 8:52AM |
M14.00004: Numerical Study of Interactions between Surface Waves and Turbulence Underneath Using Phase-Resolved Simulations Anqing Xuan, Lian Shen It has been known that waves can substantially modify the turbulent flow underneath, for example, leading to Langmuir turbulence. To elucidate the effects of surface waves on turbulence, we perform direct numerical and large-eddy simulations using a dynamically-evolving wave-surface-fitted grid with fully nonlinear kinematic and dynamic free-surface boundary conditions. Our simulations have the capability of explicitly resolving the distortion of turbulence by instantaneous phase-resolved wave motions, in addition to the averaged Stokes drift effects. The numerical results show that the effects of waves on turbulence have been successfully captured in our simulations. In the cases of wind-driven shear turbulence interacting with waves, the transition from shear turbulence to Langmuir turbulence occurs in our simulations as the turbulent Langmuir number $\mathrm{La}_{\mathrm{t}}=(u_{*}/u_{s})^{1/2}$, the ratio of the friction velocity to the surface Stokes drift velocity, is decreased. Counter-rotating vortices and enhancement of the mixing due to Langmuir circulations have been observed. Our simulations also reveal detailed information on the underlying mechanisms of the interactions between turbulence and surface waves. [Preview Abstract] |
Tuesday, November 22, 2016 8:52AM - 9:05AM |
M14.00005: Vertical vorticity at a free surface. Paul W. Fontana The concept of surface vorticity is developed as a necessary consequence of the discontinuity of flow at the fluid surface. The construct provides the proper boundary conditions for a vortex-dynamical description of surface waves. It is shown that the perturbed free surface in general possesses vertical vorticity, even when the underlying flow is irrotational and the fluid is ideal. This resolves a paradox pointed out by Umeki, who discovered irrotational surface waves with surface rotation in the horizontal plane [Phys. Fluids A {\textbf{4}}, 1968 (1992)]. A dynamical equation for vertical vorticity at the free surface is derived and interpreted physically. The traditional idea that vortex lines terminate at fluid boundaries is shown to be unphysical and is amended to include surface vorticity. The extension of vertical surface vorticity into the bulk is connected with particular topological structures, such as plughole vortices, breaking waves, and Klein’'s Kaffeel{\"{o}}ffel. This analysis generalizes boundary-layer vorticity theory to the free surface in the ideal limit. The analogy between surface vorticity on an ideal liquid and sheet currents at the surface of a superconductor is described. [Preview Abstract] |
Tuesday, November 22, 2016 9:05AM - 9:18AM |
M14.00006: ABSTRACT WITHDRAWN |
(Author Not Attending)
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M14.00007: Very-large-scale coherent motions in open channel flows. Qiang Zhong, Fazle Hussain, Dan-Xun Li Very-large-scale coherent structures (VLSSs) - whose characteristic length is of the order of 10$h$ ($h$ is the water depth) - are found to exist in the log and outer layers near the bed of open channel flows. For decades researchers have speculated that large coherent structures may exist in open channel flows. However, conclusive evidence is still lacking. The present study employed pre-multiplied velocity power spectral and co-spectral analyses of time-resolved PIV data obtained in open channel flows. In all cases, two modes - large-scale structures (of the order of $h)$ and VLSSs - dominate the log and outer layers of the turbulent boundary layer. More than half of TKE and 40{\%} of the Reynolds shear stress in the log and outer layers are contributed by VLSSs. The strength difference of VLSSs between open and closed channel flows leads to pronounced redistribution of TKE near the free surface of open channel flows, which is a unique phenomenon that sets the open channel flows apart from other wall-bounded turbulent flows. [Preview Abstract] |
Tuesday, November 22, 2016 9:31AM - 9:44AM |
M14.00008: Dynamics of motile micro-organisms in stably-stratified turbulence Salvatore Lovecchio, Francesco Zonta, Cristian Marchioli, Alfredo Soldati Motile micro-organisms populating terrestrial water bodies swim upward towards the air-water interface to capture light and activate photosynthesis. These micro-organisms have the center of mass displaced below the center of buoyancy and are usually called gyrotactic swimmers. Gyrotactic swimmers (which are almost neutrally-buoyant) are extremely sensitive to the local flow field, which is often stably stratified (due to solar heating at the water surface). Stable stratification has a deep influence on the transport processes of mass, momentum, heat and chemical species at the water surface. In this work we use Direct Numerical Simulation (DNS) and Lagrangian Particle Tracking (LPT) to analyze the dynamics of gyrotactic swimmers in stratified turbulence. Our results show that swimmers surfacing and clustering at the surface depend strongly on the re-orientation time of swimmers and on the level of stratification. Obtaining accurate predictions of the surfacing time for gyrotactic swimmers is extremely important to estimate the global $CO_2$ exchange across the air-water interface. [Preview Abstract] |
Tuesday, November 22, 2016 9:44AM - 9:57AM |
M14.00009: Micro-swimmer dynamics in free-surface turbulence subject to wind stress Cristian Marchioli, Salvatore Lovecchio, Alfredo Soldati We examine the effect of wind-induced shear on the orientation and distribution of motile micro-swimmers in free-surface turbulence. Winds blowing above the air-water interface can influence the distribution and productivity of motile organisms via the shear generated just below the surface. Swimmer dynamics depend not only by the advection of the fluid but also by external stimuli like nutrient concentration, light, gravity. Here we focus on gyrotaxis, resulting from the gravitational torque generated by an asymmetric mass distribution within the organism. The combination of such torque with the viscous torque due to shear can re-orient swimmers, reducing their vertical migration and causing entrapment in horizontal fluid layers. Through DNS-based Euler-Lagrangian simulations we investigate the effect of wind-induced shear on the motion of gyrotactic swimmers in turbulent open channel flow. We consider different wind directions and swimmers with different reo-rientation time (reflecting the ability to react to turbulent fluctuations). We show that only stable (high-gyrotaxis) swimmers may reach the surface and form densely concentrated filaments, the topology of which depends on the wind direction. Otherwise swimmers exhibit weaker vertical fluxes and segregation at the surface. [Preview Abstract] |
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